ObjectiveThis study was designed to identify the 58-kDa inner ear protein against which the sera of some patients with idiopathic, progressive sensorineural hearing loss or Ménière's disease strongly react.BackgroundWe and other groups have previously demonstrated that a 58-kDa protein extracted from guinea pig or bovine inner ear tissue is a target of antibodies in serum samples from some patients with autoimmune inner ear diseases.MethodsAfter separation of inner ear proteins by 10% sodium dodecyl sulfate polyacrylamide gel electrophoresis, the bands corresponding to 58 kDa were localized and excised from the gel. The concentrated protein was then digested with trypsin, and the peptide fragments were separated by high-pressure liquid chromatography. Three fractions were subjected to amino acid sequencing by the classic Edman degradation.ResultsThe sequence of a stretch of 14 amino acids of the first fragment was identical to that of amino acids 526 to 539 of the COCH5B2 protein. The sequences of 11 and 10 amino acids of the second and third fragments, respectively, also were identical to residues 417 to 427 and 396 to 405 of the COCH5B2 protein. These data, together with two-dimensional gel electrophoresis followed by Western blot experiments, confirmed that the 58-kDa inner ear protein is the COCH5B2 protein.DiscussionThese findings indicate that the 58-kDa target protein of antibodies in serum samples of patients with autoimmune inner ear diseases is the COCH5B2 protein, a molecule that is highly and specifically expressed in the cochlea and vestibule.